Pressure effect on magnetism and multiferroicity in ${\text{Mn}}_{2}$${\text{GeO}}_{4}$

Pressure effect on magnetism and multiferroicity in ${Mn}_{2}$ ${GeO}_{4}$

T. Honda, T. Aoyama, J. S. White, Th. Strässle, L. Keller, M. Kenzelmann, F. Honda, A. Miyake, K. Shimizu, Y. Wakabayashi, and T. Kimura

Phys. Rev. B 89, 104405 – Published 10 March 2014

Abstract

The effect of high pressure exceeding 6 GPa on magnetism and multiferroicity was investigated for the olivine ${Mn}_{2}$ ${GeO}_{4}$ that shows successive magnetic transitions at ambient pressure and a ferroelectric ground state driven by spin-spiral order. We measured heat capacity, dielectric constant, and electric polarization at various pressures using a diamond anvil cell. The pressure evolution of the magnetic structures was also investigated by powder neutron diffraction measurements using a Paris-Edinburgh press. We found that all of the magnetic transition temperatures are enhanced monotonically by applying pressure. Furthermore, the spin-driven ferroelectricity persists up to about 6 GPa but suddenly vanishes by 6.3 GPa. A consistent description of all data is achieved if an incommensurate-commensurate phase transition occurs with pressurization and causes the suppression of ferroelectricity. We discuss the origin of the observed pressure effects on the magnetism and multiferroicity in ${Mn}_{2}$ ${GeO}_{4}$ in terms of a pressure-induced change in the superexchange magnetic interactions.

Received 24 December 2013

DOI:https://doi.org/10.1103/PhysRevB.89.104405

Authors & Affiliations

T. Honda¹, T. Aoyama¹, J. S. White^2,3, Th. Strässle², L. Keller², M. Kenzelmann⁴, F. Honda⁵, A. Miyake^6,*, K. Shimizu⁶, Y. Wakabayashi¹, and T. Kimura¹

¹Division of Materials Physics, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan
²Laboratory for Neutron Scattering, Paul Scherrer Institute, CH-5232 Villigen, Switzerland
³Laboratory for Quantum Magnetism, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
⁴Laboratory for Developments and Methods, Paul Scherrer Institut, CH-5232 Villigen, Switzerland
⁵Institute for Materials Research, Tohoku University, Oarai, Ibaraki 311-1313, Japan
⁶KYOKUGEN, Osaka University, Toyonaka, Osaka 560-8531, Japan

^*Present address: Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan.

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Vol. 89, Iss. 10 — 1 March 2014

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Figure 1
A schematic of the crystal structure of MGO, Ge ions are omitted for clarity. Five important Mn-O-Mn superexchange interaction paths are shown: two intrachain superexchange interactions γ and γ′ and three interchain interactions α, α′, and α $^{''}$ . Black solid lines marking Mn1-O1-Mn1 and Mn1-O2-Mn1 paths denote the intrachain superexchange interactions γ, and gray solid lines marking Mn1-O1-Mn2 and Mn1-O3-Mn2 paths denote the intrachain interactions γ′. The green dashed line connecting Mn2-O3-Mn2 shows the interchain superexchange interaction α, the gray dashed line connecting Mn1-O2-Mn2 shows the interchain interaction α′, and the black dashed line connecting Mn1-O3-Mn2 shows the interchain interaction α $^{''}$ . The bond angles of γ and γ $^{'}$ are near the right angle; thus, these intrachain superexchange interactions are expected to be weaker than the interchain superexchange interactions α, α $^{'}$ , and α $^{''}$ with bond angles of about 130°.
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Figure 2
$T$ proﬁles of (a) $C_{ac}$ and (b) $ɛ_{c}$ measured at selected pressures. Black inverted triangles show phase transition points. Inset of (a): microscope image of MGO with four measurement wires and inside the pressure chamber. Inset of (b): a magnified view of $ɛ_{c}$ at 4.7, 5.9, and 6.3 GPa in the $T$ range from 7 to 11 K. The respective data are vertically offset for clear comparison.
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Figure 3
$T$ proﬁles of (a) pyroelectric current along the $c$ axis and (b) $P_{c}$ measured at 1 T under various pressures. $E$ and $μ_{0} H$ ﬁelds were applied parallel to the $c$ axis. These data were taken upon sweeping $T$ up at a rate of 10 K/min.
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Figure 4
PND profiles measured at 80, 20, 10, and 4.5 K under a pressure of 1 GPa. Numbers listed below the proﬁles denote general hkl indices of the respective magnetic Bragg peaks. Dot symbols and black lines represent observed data points and Le Bail fits, respectively. Cross marks denote 110 and 001 magnetic Bragg peaks, while square symbols show 100 and 300 peaks. Star symbols denote peaks due to the IC structure. The respective data are vertically offset for clear comparison.
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Figure 5
PND profiles measured at 4.5 K under pressures of 1.0, 2.7, and 5.1 GPa. Dot symbols and black lines represent observed data points and Le Bail fits, respectively. Blue triangles denote peaks due to the IC structure. The respective data are vertically offset for clear comparison.
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Figure 6
Pressure- $T$ phase diagram of MGO. Circle and triangle symbols represent data obtained by measurements of heat capacity and dielectric constant, respectively. A shaded area shows the phase where ferroelectricity exists. A black line is a fit to the $T_{N 1}$ points and is described by the formula that pressure is proportional to ln $T_{N 1}$ (see text for details). Closed symbols denote $p$ - $T$ points, where PND measurements were conducted; green, red, and blue symbols show the magnetic structures in the AFM1, AFM2, and AFM3 phases, respectively. The filled black symbol in the lowest phase at 5.1 GPa denotes a PND measurement where a C magnetic structure was observed in the AFM3 phase, but the IC structure was not (see Fig. 5).
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Physical Review B

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Pressure effect on magnetism and multiferroicity in ${Mn}_{2}$ ${GeO}_{4}$

T. Honda, T. Aoyama, J. S. White, Th. Strässle, L. Keller, M. Kenzelmann, F. Honda, A. Miyake, K. Shimizu, Y. Wakabayashi, and T. Kimura

Phys. Rev. B 89, 104405 – Published 10 March 2014

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Physical Review B

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Pressure effect on magnetism and multiferroicity in Mn2GeO4

T. Honda, T. Aoyama, J. S. White, Th. Strässle, L. Keller, M. Kenzelmann, F. Honda, A. Miyake, K. Shimizu, Y. Wakabayashi, and T. Kimura

Phys. Rev. B 89, 104405 – Published 10 March 2014

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Pressure effect on magnetism and multiferroicity in ${Mn}_{2}$ ${GeO}_{4}$